The invention provides for a pitch controlled wind turbine blade, characterized in that, said blade comprise turbulence generating means wherein said means are placed on the leeward surface side of said wind turbine blade and at the outer section of said wind turbine blade in direction of the blade tip.
It is advantageous to provide a pitch controlled wind turbine blade with turbulence generating means, in that it makes it possible to continuously optimise the blades angle of attack in relation to power output and noise emission.
Furthermore, the human ear is sensitive to noise in a narrow frequency band. Providing the blade with turbulence generating means broaden the frequency band at which the noise is emitted, making it less irritating to the human ear.
Further the turbulence generating means may transform a part of the generated noises frequency, from low frequency to high frequency. This is advantageous, in that air can much more efficiently absorb high frequency noise than low frequency noise, which thereby reduces the overall noise emission from the blade and thereby from the wind turbine on which the blade is mounted, as perceived by the human ear.
The term “leeward surface sides” may also be understood as the suction side i.e. the side of the blade facing the tower on an upwind wind turbine during normal operation.
In an aspect of the invention, said turbulence generating means are airflow noise reduction means.
Wind turbines produce noise when the blades rotate through the air. It is therefore advantageous to provide the blades with airflow noise reduction means, even though it might increase drag and thereby decrease the wind turbines efficiency marginally.
In an aspect of the invention, said turbulence generating means are placed in an outer section ranging from 60% to 100% of the length of said blade e.g. the outermost third of said wind turbine blade.
The noise emission from a wind turbine blade increases, when the rotation speed of the blade increases; and the speed of blade, relative to the surrounding air, increases towards the tip of the blade. The present range therefore provides for an advantageous placement of the turbulence generating means in relation to noise emission.
In an aspect of the invention, said outer section is subdivided into two or more subsections, and said turbulence generating means is substantially uniform within each of said subsections.
Making the turbulence generating means uniform is advantageous production wise, in that they can be mass-produced, and thereby reducing the cost of the individual turbulence generating means considerably. But to efficiently reduce the noise emission from the blade, it can be advantageous to vary the design or the size of the turbulence generating means throughout the section of the blade at which they are distributed. By subdividing the outer section into two or more subsections in which the turbulence generating means are substantially uniform, an advantageous relation between manufacturing costs and reduction of noise emission is reached.
In an aspect of the invention, the height of said turbulence generating means is of equal extent or is higher closest to the trailing edge of said wind turbine blade, than closest to the leading edge of said wind turbine blade.
Making the turbulence generating means with a constant height or higher at the back as seen from the tip or the root of the blade is advantageous, in that it provides the means with good turbulence generating qualities.
In an aspect of the invention, the highest height of said turbulence generating means is between 0.01% and 5%, preferably between 0.1% and 3% and most preferred between 0.2% and 0.8% of the chord length of said wind turbine blade.
The present height range provides the turbulence generating means with an advantageous relation between noise emission and drag.
In an aspect of the invention, the distance between said turbulence generating means are substantially constant.
Keeping the distance between the turbulence generating means substantially constant is advantageous in that it makes it easy to mount them on a ready made wind turbine blade. Furthermore it can also improve the turbulence generating means efficiency in relation to reducing noise emission.
In an aspect of the invention, said turbulence generating means are placed in a range between 5% and 85%, preferably between 10% and 75% and most preferred between 15% and 60% of the chord length, from the leading edge of said wind turbine blade.
The present range provides for an advantageous placement of the turbulence generating means in relation to noise emission.
In an aspect of the invention, said turbulence generating means are attached to the wind turbine blade individually or as pairs by means of attachment means such as screws, bolts, rivets, welding or preferably adhesive.
It is advantageous to attach the turbulence generating means individually or as pairs, in that it enables the possibility of positioning the means individually for optimal noise reduction.
In an aspect of the invention, said turbulence generating means are attached to the wind turbine blade as part of a string of tape, a coil or a band by means of attachment means such as screws, bolts, rivets, welding or preferably adhesive.
It is advantageous to attach the turbulence generating means to the surface of the blade as part of a string of tape, a coil or a band, in that it provides for a simple and cost efficient way of attaching the means.
In an aspect of the invention, said turbulence generating means are plates extending in an angle between 60° and 120° e.g. orthogonally from the surface of said wind turbine blades leeward surface side.
Designing the turbulence generating means as plates extending from the blades suction surface side in the present angle range, provides for an advantageous design of the turbulence generating means in relation to noise emission.
In an aspect of the invention, said turbulence generating means comprises sides alternately positioned in an angle in relation to the direction of the airflow between 50° and 2°, preferably between 30° and 5° and most preferred between 20° and 10° and −50° and −2°, preferably between −30° and −5° and most preferred between −20° and −10°.
Making the angle of the sides of the turbulence generating means alternately positive and negative in relation to the direction of the incoming airflow, provides for an advantageous design of the turbulence generating means in relation turbulence generation and noise emission.
In an aspect of the invention, said turbulence generating means are formed integrally with the wind turbine blade.
Forming the turbulence generating means integrally with the wind turbine blade e.g. during the manufacturing of the blade is advantageous, in that it provides for a simple and cost efficient way of providing the blade with turbulence generating means.
In an aspect of the invention, said turbulence generating means are vortex generators.
Using vortex generators as means for preventing or minimizing stall are well known within the art of making wind turbine blades, airplane wings and other. A lot of issues regarding production methods, attachment methods and other have therefore already been dealt with, and it is therefore advantageous to make use of these experiences in making turbulence generating means.
In an aspect of the invention, said wind turbine blade comprise at least one pitch controlling unit.
The invention further provides for a wind turbine comprising at least two pitch controlled wind turbine blades and pitch controlling means for pitching said blades characterized in that said blades comprise turbulence generating means wherein said means are placed on the leeward surface sides of said wind turbine blades and at the outer section of said wind turbine blades in direction of the blade tips. Hereby is achieved an advantageous apparatus according to the invention.
In an aspect of the invention, said wind turbine is a variable speed pitch controlled wind turbine.
Variable rotation speeds produces variable noise at variable levels which can be particularly irritating to the human ear. It is therefore advantageous to reduce the sound emission by use of turbulence generating means on the blades in variable speed pitch controlled wind turbine.
The invention also relates to use of pitch controlled wind turbine blades according to any of claims 1 to 15 in connection with a pitch controlled wind turbine according to any of claims 16 to 18 for adjusting blade pitch and noise level.
In a further aspect of the invention the use involves control of said noise level on the basis of noise level measurements by sound measurement means such as microphones.